The present invention relates to a method and an apparatus for controlling the exposure of video cameras. More particularly, the invention relates to a method and an apparatus for controlling an exposure amount in accordance with a video signal from an image sensor.
A conventional video camera includes a built-in exposure control apparatus which automatically regulates the intensity of light incident to an image sensor of CCD or MOS type FETs in accordance with the brightness of a scene For instance, in an apparatus for automatically adjusting an exposure amount by using a diaphragm (an iris diaphragm), the diaphragm is adjusted to an aperture size suitable for a proper exposure amount in accordance with the scene brightness value detected through averaged brightness measuring, spot brightness measuring, or multi-pattern measuring. Instead of the exposure being controlled by a diaphragm, it is also known to control a charge accumulation time of an image sensor in accordance with a scene brightness value When a video camera or a still video camera is in a continuous pick-up mode having such an exposure control apparatus, the diaphragm aperture size or the charge accumulation time may be controlled so as to quickly follow a change in a scene brightness value Thereby, the exposure control quickly responds to the start of picking up a scene or the change of a scene
Even for the case when the same principal subject is continuously picked up, the brightness distribution of a scene may change more or less. For instance, the brightness distribution of a scene changes greatly during panning while a moving principal subject is picked up. In such a case where the conventional exposure control apparatus has a quick response performance, the diaphragm aperture size or the image sensor charge accumulation time frequently changes, and an unnatural exposure control results. For instance, in the case of the averaged brightness measuring, although the brightness of a principal subject changes very little, the background brightness changes considerably and the exposure control is executed correspondingly. As a result, the exposure amount of the principal subject changes frequently so that the image brightness of the reproduced principal subject changes too often and is very awkward.
Such problems are particularly conspicuous for the case of spot brightness measuring and multi-pattern measuring. For instance, if an image of a principal subject moves away from the spot photometric area at the center of a scene because of tracking with delay for the principal subject during panning, or if an abnormal light is incident to the photometric area, the exposure control apparatus responds to the conditions and corrects the exposure. Thereby, in a change of the principal subject irrespective of a continuous scene thereof, an unnatural exposure control results.
There is also known an exposure control method by which a frame is divided into a central area and a peripheral area and either the center weighted measuring mode or the averaged measuring mode is automatically selected to obtain an optimum exposure of, e.g., a rear light scene. With such a two mode switching method and multi-pattern measuring method, the center weighted measuring is selected if the center of a scene is far brighter than the peripheral areas. Such a scene has a proper exposure for a principal subject if the subject is located at the center of the frame, but the scene becomes under-exposed if the subject is located at the peripheral area.